1. Field of the Invention
The invention relates generally to accessing a dataset, and more particularly to accessing a dataset using an unsupported access method.
2. Description of the Related Art
In a computer system, databases allow a user to store and access data quickly and conveniently. The data in a database is stored in one or more tables. Typically, the computer system stores the data as records in a table and the table is stored as a dataset on a hard disk drive. A dataset is also referred to as a file. Although disk drives can store large amounts of data, disk drives still have a limited amount of storage space. Datasets are also allocated a maximum amount of space for storing data. To prevent a dataset from running out of space, some databases store portions of the data of a dataset in archive storage. Accessing data in archive storage is typically slower than accessing data on a hard disk drive.
A log is a collection of records that describe the events that occur during database execution and that indicate the sequence of the events. The log is used to recover from a failure during database execution. An active log is a portion of the database log to which log records are written as they are generated, in particular, the log records are written when committed. An archive log is a portion of the database log that contains log records that have been copied from an active log. The active log contains the most recent log records, and the archive log contains those log records that are older and no longer fit in the active log.
Archive logs consume a large amount of storage and can create storage problems. Typically archive logs are stored on tape. Reading data from the archive logs can be time consuming, especially when waiting for a device to become available.
Some applications have such a high volume of transactions that archiving cannot keep pace with the rate at which transactions are logged. If archiving cannot keep pace with the active logging rate, then the database management system may halt until records containing the transactions are written to the archive log.
Data striping is a technique that distributes the data records of a dataset across multiple system-managed volumes, where each volume is a separate disk drive. When data is written to a striped dataset, the data is interleaved among the stripes. The system distributes the records in each of the stripes in such a way that the volumes can be read from or written to simultaneously, thereby reducing the amount of time to read and write data and improving performance.
One database management system, the IBM® DB2® (IBM and DB2 are registered trademarks of International Business Machines Corporation) database management system, uses the basic direct access method for writing data to and reading data from archive logs. In one storage system, data striping requires extended format physical sequential datasets. However, extended format physical sequential datasets are accessed using another access method which may be the basic sequential access method (BSAM) or the queued sequential access method (QSAM). Because the database management system's archive logs use the basic direct access method (BDAM) which does not support extended format physical sequential datasets, data striping cannot be used with conventional DB2 archive logs.
Therefore, there is a need for a technique to access a dataset using an unsupported access method. In particular, this technique should allow an extended format physical sequential dataset to be accessed using the basic direct access method.